2. Draw the speed-torque characteristics of dc shunt motor and series motor. (May2013) (May 2014)

Size: px

Start display at page:

Download "2. Draw the speed-torque characteristics of dc shunt motor and series motor. (May2013) (May 2014)"

Stephany Parrish
6 years ago
Views:

1 UNIT 2 - DRIVE MOTOR CHARACTERISTICS PART A 1. What is meant by mechanical characteristics? A curve is drawn between speed-torque. This characteristic is called mechanical characteristics. 2. Draw the speed-torque characteristics of dc shunt motor and series motor. (May2013) (May 2014) 3. A series motor should never be started without some mechanical load why? When the load current Ia falls to a small value, speed becomes dangerously high. Hence a series motor should never be started without some mechanical load. 4. What is rheostatic braking? In rheostatic braking the armature is disconnected from the supply and is connected as a variable resistance. The braking is controlled by varying the series resistance. 5. What are the different types of dc motor? I. DC series motor 2. Shunt motor 3. Compound motor 4. Separately excited de motor 6. What is meant by electrical characteristics? A curve is drawn torque and armature current. characteristics. It is known as electrical 7. What is the relation between speed and flux of a dc motor? The speed of a dc motor is inversely proportional to field flux.

2 8. What is the application of dc motor? DC shunt motor:- 1. For driving constant speed operations 2. Machine tools 3. Lathes 4. Blowers and fans 5. Centrifugal pumps 6.Reciprocating pumps DC series motor:- 1.Electric locomotives 2. Rapid transit systems3.trolley cars4.cranes and hoists5. Conveyors DC compound motor:-1. Elevators.2. Air compressors3. Rolling mills4. Heavy planers 9. A dc shunt motor is called as constant speed motor-why? ( May 2015) The drop in speed from no-load full-load is small; hence the dc shunt motor is also called as constant speed motor. 10. What is mean by braking? Whenever an electric drive is disconnected from the supply, the speed of the driving motor gradually decreases and becomes zero. Braking is a generic term used to describe a set of operating conditions for electric drive systems. It includes rapid stopping of the electric motor holding the motor shaft to a specific position, maintaining the speed to a desired value of preventing the motor from over speeding. 11. What are the two types of braking? 1. Mechanical braking 2. Electrical braking 12. What is meant by mechanical braking? In mechanical braking, the frictional force between the rotating parts and brake drums provide the required brake. 13. What is meant by electric braking? In electric braking, the motor is made to work as generator. So it produces a negative slip and negative torque (braking torque). This is achieved by suitably changing the electrical connections of the motor. 14. What are the different types of electric braking?(nov2013)(may 2013) ( May 2015) 1. Regenerative braking 2. Dynamic braking 3. Plugging 15. What are the advantages of electric braking?

3 1. High efficient method 2. Low maintenance 3. Braking is very smooth 16. What is meant by regenerative braking? (Dec 2011) In the regenerative braking operation, the motor operators as a generator, while it is still connected to the supply. Here, the motor speed is greater than the synchronous speed. Mechanical energy is converted into electrical energy, part of which is returned to the supply and rest of the energy is last as heat in the winding and bearings. 17. What is meant by dynamic braking? When an electric motor rotates, a kinetic energy of the motor is converted into electric energy. This energy is dissipated in resistive elements. 18. What is meant by plugging? (Dec 2014) The plugging operation can be obtained by changing the polarity of the motor. For a machine, the phase sequence of the starter windings and dc machines the polarities of the field or armature terminals. 19. What are the disadvantages of dc machine? 1. Higher cost 2. Higher rotor inertia 3. EMI problems 4. Maintains problems with commutator and brushes 5. Do not permit a machine to operate in dirty and explosive environments. 20. What are the advantages of squirrel cage induction motor? 1. Rugged 2. Cheaper 3.Lighter 4.More efficient 5. Less maintenance6. Can operate in explosive and dirty environment. 21. What are the two types of rotors in three phase induction motors? 1. Squirrel cage rotor 2. Slip ring rotor 22.What is the necessity of braking?(dec 2014) The quickness and accuracy of braking techniques determine the productivity and quality of the manufactured goods.control the motor for our optimum requirement. 23.What are different methods of Braking of DC series motor? (Nov 2015) 1. Regenerative braking 2. Dynamic braking 3. Plugging

4 24. A 220V DC shunt motor having the armature current of 10A, runs at 1500 rpm. Find the armature current if the source voltage drops to 150V. Assume the load torque is constant.(nov 2015) Given, V1=220V, Ia1=10A, V2=150V, Ia2=?. For Shunt motor, Ia1Φ1= Ia2Φ2 (since torque is constant) Φ2=(150/220) Φ > Φ2=0.68 Φ1. Therefore, Ia2=(10 Φ1/0.68 Φ1) = 14.67A. PART B 1. Explain about the speed-torque characteristics of a DC Compound Motor with suitable graph and equations.(dec 2013) The DC motor that has both shunt as well as series field winding is known as DC compound Motor. If the flux produced by series field winding helps (aids) the flux produced by shunt field, then the total flux increases and DC compound motor is known as Cumulative Compound Moor. On the other hand, if the series field flux opposes the Shunt field flux, total flux decreases and the motor is known as Differential compound Motor. (N VsIa), (T VsIa) and (N Vs T) characteristics should be obtained. 2. Draw and explain the speed torque characteristics of dc series motor and three phase induction motor.(dec 2011) The DC motor in which the field winding is connected in series with Armature Conductors is known as DC series Motor. In series circuit the current flowing through the armature is same as the current flowing through series field winding. Since the full load armature current flow through series field, the magnetic saturation takes place (Flux don t increase with current after certain point ).(N VsIa), (T VsIa) and (N Vs T) characteristics should be obtained. 3. Explain about the speed-torque characteristics of a DC Shunt Motor with suitable graph and equations. (Dec 2014) Speed / Torque Characteristics of DC shunt Motor DC Shunt Motor The DC Motor is which in Field Winding is connected in Parallel with Armature conductors is known as DC Shunt motor since Field winding is connected in parallel with both supply and the armature, the voltage across the field winding is always constant. So the current flowing through the field winding and hence the flux produced in the DC shunt Motor is Constant.

5 (i) Speed / Current Characteristics (N VsIa) (ii) Torque / Current Characteistics (T VsIa ) (iii) Speed / Torque Characteristics ( NVs T)

6 4. Explain about the quadrant diagram of speed-torque characteristics for a motor driving hoist load.(nov 2015) (May 2015) Theory : Explanation in detail for the 4 quadrant is needed.

8 5. Explain how an induction motor is brought to stop by (i) Plugging and (ii) dynamic braking. (Dec 2014) (a) Plugging or Reverse Current Braking of 3 Induction Motor Plugging incase of 3 induction motor is obtained by interchanging any of the two phases, out of the three phases. There are three phase terminals in 3 induction motor and it is represented by R, Y and B. The order in which, these phases progresses is known as Phase sequence. The phase sequence determines the direction of rotation and torque. The normal working induction motor is shown in Fig. The phase sequence applied is RYB and the motor rotates in Clockwise direction. The Plugging or Reverse Current Braking is obtained by interchanging supply phase terminals Y and B. Now the phase sequence is changed to RBY. The change is phase sequence from RYB to RYB changes the direction of current and hence the direction of rotating magnetic field. Due to this, direction of torque is reversed and this torque is known as Braking torque. The Braking torque reduced the speed of the motor to standstill. If the supply terminal is not disconnected at this stage, then the braking torque accelerates the motor in the anit clockwise direction. So plugging is accomplished by changing the supply terminals. The reverse current braking characteristic is shown in Fig.. It has two part namely normaly motoring zone before the application of plugging. The Slip = 0 corresponds to Synchronous speed Ns. The induction motor always runs at speed less than synchronous speed. When reverse current braking is applied the braking torque is generated and is shown as braking zone in Fig. The Speed reduced to zero i.e., n = 0 and the corresponding Slip, S = 1

9 b.dynamic or Rheostatic braking (3 Slip Ring Induction Motor) Rheostatic braking is not possible in case of squirrel cage induction motor and is only possible in the case of slip ring induction motor. Here braking is obtained by exciting any of the two stator phase with DC supply and rotor circuit is connected to star connected external rheostat. The braking is accomplished by disconnecting the stator form 3 supply and connecting any of the two phases with DC supply. Simultaneously external star connected rheostat is connected to the rotor circuit through slip rings. Now the start produced constant flux due to DC supply and the rotor is still rotating due to inertia. So emf is induced in the rotor and current flows in such a direction to oppose the cause (speed). So braking torque is produced that results in, stoppage of the motor. The principle of rheostatic braking can also arrived by the fact that the kinetic energy of rotating

10 mass is converted into electrical energy in rotor and it is wasted in star connected rheostat and the motor stops, because the kinetic energy is consumed. From the braking characteristics it is clear that braking torque become zero, when the motor is stopped i.e there is no holding torque. The magnitude of braking torque is varied by changing the resistance of the star connected rheostat. 6. Explain the various methods of braking of induction motors. (June 2014) Electrical braking Methods of AC motor (Induction motor) The braking methods that are applied to DC motors are also applied to AC motors, particularly 3 induction motor. The braking methods are 1. Plugging or Reverse Current Braking 2. Dynamic or Rheostatic Braking and 3. Regenerative Braking 7. Describe the speed Torque characteristics of DC Dynamic braking of three phase induction motor. (May 2014)

11 Rheostatic braking is not possible in case of squirrel cage induction motor and is only possible in the case of slip ring induction motor. Here braking is obtained by exciting any of the two stator phase with DC supply and rotor circuit is connected to star connected external rheostat. The braking is accomplished by disconnecting the stator form 3 supply and connecting any of the two phases with DC supply. Simultaneously external star connected rheostat is connected to the rotor circuit through slip rings. Now the start produced constant flux due to DC supply and the rotor is still rotating due to inertia. So emf is induced in the rotor and current flows in such a direction to oppose the cause (speed). So braking torque is produced that results in, stoppage of the motor. The principle of rheostatic braking can also arrived by the fact that the kinetic energy of rotating mass is converted into electrical energy in rotor and it is wasted in star connected rheostat and the motor stops, because the kinetic energy is consumed. From the braking characteristics it is clear that braking torque become zero, when the motor is stopped i.e there is no holding torque. The magnitude of braking torque is varied by changing the resistance of the star connected rheostat.

12 8. Explain speed- torque characteristics of different types of load with graph.(may 2013) N Vs T characteristics of shunt, series and compound motor should be explained. 9. A 220 V dc series motor runs at1200 rpm (clockwise) and takes an armature current of 80 A when driving a load with constant torque. Armature resistance is 0.05Ω and field resistance is 0.05Ω. Find the magnitude and direction of motor speed and armature current if the motor terminal voltage is reversed and number of turns in field winding is reduced to 80%. Assume linear magnetic circuit. (Dec 2013) Answer: Direction of current is anticlockwise. Eb1 = 212V; Ia2 = 100A; Eb2 = 210V; N2 = 1485 rpm. 10. Explain various methods of braking of DC Series Motors with neat diagrams.(may 2013) The DC motor in which the field winding is connected in series with Armature Conductors is known as DC series Motor. In series circuit the current flowing through the armature is same as the current flowing through series field winding. Since the full load armature current flow through series field, the magnetic saturation takes place (Flux don t increase with current after certain point ). (N VsIa), (T VsIa) and (N Vs T) characteristics should be obtained. 11.Discuss the dynamic braking of DC shunt motor. (Nov 2015) Rheostatic or Dynamic Braking Rheostatic braking of DC motor is obtained by disconnecting the armature terminals from the supply and connecting it to the external resistance R. The field winding is not disturbed. Rheostatic Braking of DC shunt Motor The armature terminals are disconnected from the supply and connected to external resistance R. now the armature current direction is reversed. Due to this reversed current direction the torque is produced in the opposite direction. Which is known as braking torque. Now the armature generates voltage and hence acts as a separately exited generator. The kinetic energy of the moving system is converted into electrical

13 energy, which is dissipated in the external resistance R. Here the motor is braked by generated action. The connection diagram for Rheostatic braking of DC shunt motor shown below. 12. List the advantages and disadvantages of Electrical braking over mechanical braking. Discuss any one method of electrical braking of DC machines.(may 2015)

14 Electrical Braking methods for DC motor. The Electrical braking is applied to DC motor by generating the electrical braking torque. The braking torque is obtained by just changing some of the electrical connection of the electric motor. In electric braking the drive motor itself methods available, they are 1. Plugging or reverse current or counter current 2. Rheostatic braking 3.Regenerative braking Plugging in DC Series motor The reverse current braking of series motor is obtained by reversing the armature terminals. Due to reversal of armature terminals, the direction of the armature current is reversed. Enough care should be taken to ensure that the field current is not reversed. This is because in series motor armature and series field are in the same circuit. The braking torque is produced due to reversal of armature current. To limit the over current during braking an external resistance R is included in the armature current. The Plugging is also done in series motor by separately existing the series field winding.

15 Rheostatic braking of DC series Motor The Rheostatic braking of DC series motor is shown. Here armature and field winding present in the same circuit. Rheostatic Braking is obtained by disconnecting the armature circuit form the supply and connected to external resistance R. Now armature current is reversed. Since this is series motor, the field current is also reversed (armature current is also the field current). This leads to demagnetization of field circuit, as a result generator action do not takes place. To avoid this, field terminals (f1, f2) are reversed when armature circuit is connected to the external resistance R. Now motor acts as a generator and as the a result, kinetic energy of the moving system is converted to the electrical energy and wasted in the resistance R. Since the kinetic energy of the moving system is wasted, Braking torque is produced. Regenerative braking of DC series Motor In series motor the armature and the field, both present in the san circuit. Here the armature current is equal to the field current. When the over hauling load increase the speed of the DC motor, the back emf also increase. So the current also flow through the field windings the flux is also decreases. Due to decrease in flux, it is clear that the back emf never exceeds the supply voltage. So there is no possibility of motor entering into the generating mode. So there is no chance of regenerative braking torque. It is clear from the above discussion that Regenerative braking is not possible in case of DC series motor. The DC series motor in practice is subjected to over hauling loads, as in the case of hoist (lowering load) and traction (when descending the mountain). So the regenerative braking can be obtained in the DC series motor by separately exciting the series field winding. This method of separately exciting DC series motor to obtain

16 Regenerative braking. Now the Back emf (Eb) exceeds the supply voltage (V), as a result the machine enters into generating mode. The current direction is reversed now (towards the supply mains), as a result regenerative braking torque is produced, due to magnetic drag associated with generating mode. The speed of the machine is reduced till it reaches the rated no load speed. When the speed of the machine reaches rated no load speed, back emf is no longer greater than supply voltage and the machine reenters the motoring mode.

EE6351 ELECTRIC DRIVES AND CONTROL UNIT-1 INTRODUTION

EE6351 ELECTRIC DRIVES AND CONTROL UNIT-1 INTRODUTION 1. What is meant by drive and electric drive? Machines employed for motion control are called drives and may employ any one of the prime movers for